Python列表的转换

Question

a = [1, 1, 2, 2, 2, 3]
元素已排好序
希望变成：
b = [2, 4, 5]
即按照出现次数累加
怎么写比较简洁高效？

Answer 1

python
import collections

counter = collections.Counter([1, 1, 2, 2, 2, 3])

print counter

>>> Counter({2: 3, 1: 2, 3: 1})

Answer 2

>>> valdict = dict((k, len(list(g))) for k, g in groupby(sorted(a)))
>>>
>>> for key, val in valdict.items(): print key, ":", val
...
1 : 2
2 : 3
3 : 1